Selective IgA deficiency presents with recurrent mucosal infections. Which immunoglobulin is deficient?

• A. IgG
• B. IgA
• C. IgM
• D. IgE

Answer: (A)

Secretory IgA protects mucosal surfaces, so a deficiency in this antibody leads to problems with infections at those sites. In selective IgA deficiency, IgA is absent or very low, while IgG and IgM are normal. That loss at mucosal barriers allows recurrent mucosal infections such as sinopulmonary and GI infections even though systemic immunity remains intact because other immunoglobulins are still present. A practical note is that some individuals have anti-IgA antibodies and can have severe reactions to IgA-containing blood products, which is an important clinical consideration. IgG, IgM, and IgE have roles outside the mucosal defense affected here: IgG provides systemic protection, IgM is the initial responder in many infections, and IgE drives allergic responses and defense against parasites.